Maybe it's just me, but this seems like a great prank if you're moving offices at your job or if you're leaving...it's not doing any real damage, but it's certainly leaving a mark. Of course, if they aren't too computer savvy and don't have an IT department, this might affect your future reference when they claim you trashed a computer before you left.

There's also the so-called David Hasselhoff attack, which consists on changing the desktop to a picture of The Hoff (a good one is one where he's posing naked with some puppies blocking his privates, google it as "david hasselhoff puppies" if you're not at work / sane). A more elaborated version includes changing the security policies in the control panel afterwards so you can't cange the desktp wall paper.

The ultimate prank would include a combination of that, and the one explained in this post.

Fooled the IT guy years ago with something similar... Used a blurred version of the screen cap as the background, then removed all the desktop icons. He never noticed the mouse pointer wasn't blurred. Came close to getting a new monitor.